Field of the Invention
The present invention relates to a fixing belt, a fixing device, an image forming apparatus and an image formation method.
Description of Related Art
Normally, in fixing devices employed in image forming apparatuses such as copiers and laser beam printers, a heated fixing belt is brought into contact with a recording medium bearing an unfixed toner image to fix the toner image to the recording medium. In such fixing devices, for example, one of two or more rollers that rotatably support the fixing belt is a heating roller that applies heat to the fixing belt. Since the thermal capacity of the fixing belt is relatively small, the fixing devices provide favorable fixation performance, and are advantageous in terms of speedup of image formation, for example.
Known examples of the fixing belt include: an endless fixing belt including a viscoelastic layer, a base layer and a releasing layer which are stacked on one another in this order, and having a loss modulus of 20 to 80 GPa; and an endless fixing belt including a base layer, an elastic layer and a surface layer which are stacked on one another in this order in which a loss tangent tan δ which is a ratio of a loss modulus relative to a storage modulus at 180° C., amplitude 10 μm, and 1 Hz in the elastic layer is 0.06 to 0.2 (see, for example, Japanese Patent Application Laid-Open No. 2008-158053 and Japanese Patent Application Laid-Open No. 2005-300591).
Meanwhile, further speedup in the image formation of the image forming apparatus is demanded. In general, when the fixing rate (also referred to as “printing rate”) in an image forming apparatus is low, the tensile force of the fixing belt in the fixing device increases. The reason for this is that in a low speed image forming apparatus, rollers having a small roller diameter are used as rollers of the fixing device, and consequently, it is required to increase the tensile force to interlock the fixing belt with the rotation driving of the rollers that rotatably support the fixing belt while preventing the slip between the fixing belt and the rollers.
Normally, when the printing rate of the image forming apparatus increases, the roller diameter of the roller increases for the heat value that should be held by the roller, and consequently the tensile force of the fixing belt decreases. This reduces the ease of separation of the recording medium on which a toner image has been fixed from the fixing belt. As a result, paper jam (also referred to as “jam”) is easily caused. As such, a so-called biaxial belt fixing device in which the fixing belt that is heated by a heater is rotatably supported has a room for improvement in terms of speedup.